When I do a video call with FaceTime the phone switches to mobile data network despite strong WiFi permanently. This makes absolutely no sense because the connection is much slower. Sure, I can disable this but the reason for that behaviour is not clear to me. Disabling mobile data is no option because without WiFi I need the mobile data network for FaceTime but not with WiFi.
No such behavior with WhatsApp video calls.
iPhone XS, iOS: 13.5.1